Higher operating temperatures for gas turbine engines are sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys. While superalloys have found wide use for components in gas turbine engines, alternative materials have been proposed. Materials comprising silicon, particularly those with silicon carbide (SiC) as a matrix material and/or reinforcing material, have been considered for high temperature applications, such as combustor and other hot section components of gas turbine engines.
In many applications, a protective coating is beneficial for Si-comprising materials. For example, protection with a suitable thermal-insulating layer reduces the operating temperature and thermal gradient through the material. Additionally, such coatings may provide environmental protection by inhibiting the major mechanism for degradation of Si-comprising materials in a corrosive water-comprising environment, namely, the formation of volatile silicon hydroxide (Si(OH)4) products. Consequently, besides low thermal conductivity, a thermal barrier coating system for a Si-comprising material should be stable in high temperature environments comprising water vapor. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the Si-comprising material, low permeability for oxidants, and chemical compatibility with the Si-comprising material and silica scale formed from oxidation. As a result, suitable protective coatings for gas turbine engine components formed of Si-comprising materials have a dual function, serving as a thermal barrier and simultaneously providing environmental protection. A coating system having this dual function is often termed a thermal/environmental barrier coating (T/EBC) system.
While various single-layer and multilayer T/EBC systems have been investigated, each has shortcomings relating to the above-noted requirements and properties for compatibility with Si-comprising materials. For example, a coating of zirconia partially or fully stabilized with yttria (YSZ) as a thermal barrier layer exhibits excellent environmental resistance by itself since it does not comprise silica. However, YSZ does not adhere well to Si-comprising materials (SiC or silicon) because of a CTE mismatch (about 10 ppm/° C. for YSZ as compared to about 4.9 ppm/° C. for SiC/SiC composites). Mullite (3Al2O3.2SiO2) has been proposed as a bond coat for YSZ on Si-comprising substrate materials to compensate for this difference in CTE (mullite has a CTE of about 5.5 ppm/° C.). However, mullite exhibits significant silica activity and volatilization at high temperatures if water vapor is present.
Barium-strontium-aluminosilicate (BSAS) coatings suitable for Si-comprising materials exposed to temperatures of up to 2400° F. (about 1315° C.) have also been proposed. BSAS provides excellent environmental protection and exhibits good thermal barrier properties due to its low thermal conductivity. However, for application temperatures approaching the melting temperature of BSAS (about 1700° C.), a BSAS protective coating requires a thermal-insulating top coat. The addition of such a top coat on a BSAS bond coat can significantly increase the overall thickness of the T/EBC system. As application temperatures increase beyond the thermal capability of a Si-comprising material (limited by a melting temperature of about 2560° F. (about 1404° C.) for silicon) and the surface temperatures increase (up to 3100° F., or about 1704° C.), still thicker coatings capable of withstanding higher thermal gradients are required. As coating thickness increases, strain energy due to the CTE mismatch between individual coating layers and the substrate also increases, which can cause debonding and spallation of the coating system. Application of a top layer by EB-PVD methods on components such as airfoils results in a top coat having a columnar strain-tolerant microstructure. This helps to reduce stress and partially release strain energy, rendering the T/EBC more durable. However, high surface temperatures can cause rapid sintering of the top coat, which leads to less of the strain-tolerant microstructure and the development of horizontal and through-thickness cracks.
Under normal conditions of operation, coated turbine engine components can be susceptible to various types of damage, including erosion, oxidation, and attack from environmental contaminants. At the higher temperatures of engine operation, these environmental contaminants can adhere to the heated or hot coating surface and thus cause damage to the coating. For example, these environmental contaminants can form compositions that are liquid or molten at the higher operating temperatures of gas turbine engines. These molten contaminant compositions can dissolve the coating, or can infiltrate its porous structure, e.g., they can infiltrate the pores, channels or other cavities in the coating. Upon cooling, the infiltrated contaminants solidify and reduce the coating strain tolerance, thus initiating and propagating cracks that cause delamination, spalling and loss of the coating material either in whole or in part. The modified outer layer typically displays an increased CTE and/or increased stiffness that decreases compliance of the outer layer to the underlying environmental barrier layer.
The pores, channel or other cavities that are infiltrated by such molten environmental contaminants can be created by environmental damage, or even the normal wear and tear that results during the operation of the engine. Typically, the pores, channels or other cavities in the coating surface are the result of the processes by which the coating is deposited onto the underlying substrate. For example, coatings that are deposited by (air) plasma spray techniques tend to create a sponge-like porous structure of open pores in at least the surface of the coating. By contrast, coatings that are deposited by physical (e.g., chemical) vapor deposition techniques tend to create a porous structure comprising a series of columnar grooves, crevices or channels in at least the surface of the coating. This porous structure can be important in the ability of these coating to tolerate strains occurring during thermal cycling and to reduce stresses due to the differences between the coefficient of thermal expansion (CTE) of the coating and the CTE of the underlying substrate.
For turbine engine components having outer coatings with such porous surface structures, environmental contaminant compositions of particular concern are those comprising oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. See, for example, U.S. Pat. No. 5,660,885 (Hasz et al), issued Aug. 26, 1997, which describes these particular types of oxide environmental contaminant compositions. These oxides combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide oxide systems (Ca—Mg—Al—SiO), hereafter referred to as “CMAS.” The contaminant compositions may also comprise oxides of iron and titanium, and at lower temperatures, also sulfates of potassium, calcium and sodium. During normal engine operations, CMAS can become deposited on the coating surface, and can become liquid or molten at the higher temperatures of normal engine operation. Damage to the coating typically occurs when the molten CMAS infiltrates the porous surface structure of the coating. After infiltration and upon cooling, the molten CMAS solidifies within the porous structure. This solidified CMAS causes stresses to build within the coating, leading to partial or complete delamination and spalling of the coating material, and thus partial or complete loss of the protection provided to the underlying substrate of the component.
Accordingly, it would be desirable to protect environmental barrier coatings having a porous surface structure against the adverse effects of environmental contaminants. In particular, it would be desirable to be able to protect such environmental barrier coatings from the adverse effects of deposited CMAS.